1. Field of the Invention
The present invention relates to an optical recording and/or reproduction device, and more particularly, to an optical pickup to prevent deterioration of a tracking error signal caused by an adjacent layer during recording and/or reproduction of data onto/from a multi-layer recording medium that has a plurality of recording layers on one side.
2. Description of the Related Art
In an optical recording and/or reproducing apparatus which records information on and/or reproduces information from an optical information storage medium, such as an optical disc, by using laser light focused into a light spot by an objective lens, a recording capacity is determined by a size of the light spot. A size S of the light spot is determined by a wavelength λ of laser light and a numerical aperture (NA) of the objective lens, as shown in Equation 1:S∝λ/NA  (1)
Accordingly, in order to reduce the size of the light spot focused on the optical disc to increase recording density, a short wavelength light source such as a blue laser and an objective lens with an NA of more than 0.6 are required.
Since the emergence of a compact disc (CD) to record and/or reproduce information using light with a wavelength of 780 nm and an objective lens with an NA of 0.45 or 0.5, much research has gone into increasing information storage capacity by increasing recording density. The result of that research is a digital versatile disc (DVD) that can record and/or reproduce information using light with a wavelength of 650 nm, and an objective lens with an NA of 0.6 or 0.65.
At present, there is ongoing research into a high-density optical disc with over 20 GB of recording capacity using a blue wavelength light, e.g., light with a wavelength of 405 nm. The high-density optical disc is currently in the process of standardization, some of which is almost finalized. The standards specify use of light with a blue wavelength, for example, a wavelength of 405 nm, and an objective lens with an NA of 0.65 or 0.85, which will be described later on.
While the standard thickness of a CD is 1.2 mm, the standard thickness for a DVD is only 0.6 mm. The reason for the reduced thickness of DVDs is to ensure a tilt tolerance since the NA of the objective lens is increased from 0.45 for CDs to 0.6 for DVDs.
In other words, when θ denotes a tilt angle of an optical disc, n denotes a refractive index of the optical disc, d denotes the thickness of the optical disc, and NA denotes the NA of the objective lens, a coma aberration W31 caused by tilt of the optical disc is given by Equation 2:
                                          W            31                    =                                    -                              d                2                                      ⁢                                                                                            ⁢                                                                            n                      2                                        ⁡                                          (                                                                        n                          2                                                -                        1                                            )                                                        ⁢                  sin                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                  cos                  ⁢                                                                          ⁢                  θ                                                                              (                                                            n                      2                                        -                                                                  sin                        2                                            ⁢                      θ                                                        )                                                  5                  /                  2                                                      ⁢                          NA              3                                      ⁢                                                      (        2        )            
wherein the refractive index n and the thickness d of the optical disk denotes a refractive index and a thickness of an optical medium ranging from a light incident surface to a recording layer, i.e., a protective layer, or a substrate.
Considering Equation 2, in order to provide a tilt tolerance, the thickness of the optical disc must be reduced when the NA of the objective lens is increased for high-density recording. Therefore, in order to provide the tilt tolerance, there is a tendency to reduce the thickness d, and so while CDs are 1.2 mm thick, DVDs are only 0.6 mm thick.
Additionally, in the case of a high-density optical disc with higher storage capacity than a DVD, if the NA of the objective lens for the high-density optical disc is increased to, for example, 0.85, the thickness of the high-density optical disc must be reduced to about 0.1 mm to prevent performance deterioration caused by tilt of the optical disc. Thus, a blu-ray disc (BD) system has an objective lens with an increased NA and a much thinner optical disc. A BD standard regulates the wavelength of a light source to 405 nm, the NA of an objective lens to 0.85, and the thickness of the optical disc to about 0.1 mm.
Another type of high-density optical disc is called an advanced optical disc (AOD). An AOD standard regulates the wavelength of a light source to 405 nm, the NA of an objective lens to 0.65, and the thickness of the optical disc to about 0.6 mm.
Here, the thickness of the optical disc is a distance between a surface where light is incident from the objective lens and an information storage surface. In the case of CDs and DVDs, the thickness refers to the thickness of the substrate. In the case of BDs, the thickness may refer to the thickness of a protective layer.
In an optical disc with a reduced thickness of 0.1 mm, one of the biggest problems is reducing a thickness deviation across the disc to less than ±3 μm. This may be inferred from Equation 3 below which expresses spherical aberration W40 in terms of a thickness error Δd of the optical disc that causes the spherical aberration:
                                          W                          40              ⁢              d                                =                                                                      n                  2                                -                1                                            8                ⁢                                  n                  3                                                      ⁢                                          (                NA                )                            4                        ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            d                          ⁢                                                      (        3        )            
wherein, n denotes the refractive index of a material of the disc from a light incident surface to an information storage surface, and NA denotes numerical aperture.
FIG. 1 is a graph showing a relationship between a thickness error of an optical disc and a wavefront aberration when using a light of a wavelength λ=400 nm and an objective lens with NA=0.85. As shown in FIG. 1, when the thickness error is over ±3 μm, spherical aberration produces a wavefront aberration (OPD (λ)) over 0.03λ. Thus, in a system that uses a high NA such as 0.85, detection and/or compensation of spherical aberration is indispensable.
Meanwhile, a standard DVD dual-layer disc that records information on two layers to increase storage capacity has a distance of about 55 μmm between the two layers. In order to further increase the storage capacity of the high-density optical disc, the high-density optical disc is expected to be similarly formed with a structure having a plurality of recording layers. Here, the distance between the layers is roughly determined in direct proportion to a depth of a focus.
Since the depth of the focus is directly proportional to the relation λ/NA2 and the distance between the two layers of the DVD dual-layer disc is about 55 μm, the distance between two layers when forming a dual-layer BD may be, for example, about 17 μm. Here, a multi-layer optical disc with two or more recording layers on one side of the optical disc has a much larger recording capacity than an optical disc with a single recording layer.
Optical discs may be divided into single layer optical discs with a single recording layer on one side, and multi-layer optical discs with a plurality of recording layers on one side, according to the number of recording layers on one side. In addition, optical discs may be divided into a structure with a recording layer on only one side of the optical disc and another structure with a recording layer on both sides of the optical disc.
An optical disc with two recording layers on one side is called a dual-layer optical disc. Dual-layer optical discs may be further classified into dual-layer optical discs with a single-sided structure and dual-layer optical discs with a double-sided structure.
A differential push-pull (DPP) method to compensate for a push-pull offset produced when reproducing data from an eccentric optical disc is usually adopted for a tracking method of a recordable optical disk. Using a grating, light is diffracted into three beams. A ratio of diffracted light of negative 1st-order:0th-order:positive 1st-order is more than 1:10:1. That is, making the amount of 0th-order diffracted light large in order to increase light-use efficiency is advantageous.
FIG. 2 is a diagram of a structure of a photodetector 1 to detect a tracking error signal using the DPP method. 0th-order light is received at light receiving regions A through D, and positive and negative 1st-order light are received at light receiving regions E through H. If a phase of the positive and negative 1st-order light is shifted by 180 degrees with respect to the 0th-order light, a tracking error signal TESDPP=[(A+D)−(B+C)−κ[(E−F)+(G−H)] that is detected using the DPP method is derived, and an offset of the tracking error signal caused by movement of the objective lens is compensated for. Here, κ is 10/(1+1)=5 when the light ratio of the negative 1st-order beam, the 0th-order beam and the positive 1st-order beam is 1:10:1.
In the case of the dual-layer optical disc, L1 is assumed to be a layer closer to a light incident surface and L2 is assumed to be a layer further away. During recording and/or reproduction, light entering the photodetector is affected by not only the layer that is disposed at the focal point of an objective lens but also by the adjacent layer.
The distance between the layers as regulated in the standard is decided so that information recorded on the optical disc is not affected by cross-talk between layers. Thus, in an optical pickup, cross-talk between layers should not affect a servo signal.
FIG. 3 is a diagram of an optical path during reproduction of data from a dual-layer optical disc. Referring to FIG. 3, when reproducing from a layer L1 closer to the light incident surface, the focus of light L12 reflected from a layer L2 is located in front of the focus of light L11 reflected from the layer L1 and received by a photodetector 1. On the other hand, when reproducing from the layer L2, the focus of light L21 reflected from the layer L1 is located behind the focus of the light L22 reflected from the layer L2 and received by the photodetector 1.
FIG. 4A is a view of a distribution of light collected at the photodetector 1 when reproducing from the layer L1. FIG. 4B is a view of a distribution of light collected at the photodetector 1 when reproducing from the layer L2. In FIG. 4A, L11—0th-order light, L11_positive and negative 1st-order light, and L12—0th-order light denotes 0th-order light reflected from the layer L1, positive and negative 1st-order light reflected from the layer L1, and 0th-order light reflected from the layer L2, respectively, when reproducing from the layer L1. In FIG. 4B, L22—0th-order light, L22_positive and negative 1st-order light, and L21—0th-order light denotes 0th-order light reflected from the layer L2, positive and negative 1st-order light reflected from the layer L2, and 0th-order light reflected from the layer L1, respectively, when reproducing from the layer L2.
If the amount of 0th-order light L12 and L21 is presumed to be substantially similar as the amount of 0th-order light L11 and L22, the amount of the 0th-order light L12 and L21 corresponds to ten times the amount of 1st-order light L11 and L22. Although the amounts of the 0th-order light L12 and L21 would not actually be the same as the amounts of the 0th-order light L11 and L22, respectively, the 0th-order light may affect the 1st-order light L11 and L22. Therefore, even if the 0th-order light L12 and L21 is defocused, the 0th-order light has an effect on a tracking signal. Particularly, if the 0th-order light L12 and L21 is varied by changes in, for example, the distance between the layers, the tracking signals fluctuate.
FIG. 5 is a view illustrating the measurement signals of a difference signal (E−F) of a detection signal at the light receiving regions E and F, a difference signal (G−H) of a detection signal at the light receiving regions G and H, and a summing signal of the difference signals [(E−F)+(G−H)] when reproducing from the layer L1. As shown in FIG. 5, the fluctuations of the difference signal (E−F) and the difference signal (G−H) have opposite phases overall. Even so, when the difference signals (E−F) and (G−H) are summed, the fluctuations are not compensated for but remain.
Therefore, considering that the tracking error signal detected using the DPP method is TESDpp=[(A+D)−(B+C)−κ[(E−F)+(G−H)], the tracking error signal detected using the DPP method fluctuates by, for example, the changes in the distance between the layers.